1. Field of the Invention
The present invention relates to methods and systems for exposing a target using charged particles, and methods and systems for processing exposure data for use in charged particle lithography, and in particular to methods and systems for performing stitching during exposure of a target.
2. Description of the Related Art
In a multi-beam lithography system, multiple beams are used to expose a target, such as a silicon wafer coated with resist. Charged particle beams, such as electron beams, are usually used. The multiple beams are scanned across the surface of the target, each beam simultaneously writing a portion of the pattern onto a portion of the target. To provide the required precision at a satisfactory throughput, a very large number of beams may be used, e.g. tens or hundreds of thousands, or even millions of beams. An example of such a system is described in M. J. Wieland et al, “Throughput enhancement technique for MAPPER maskless lithography”, Proc. of SPIE, Vol. 7637, 76371Z (2010).
During a single exposure the lithography system usually exposes an area of the target, e.g. a single 26 mm×33 mm field. Each beam is used to scan a certain allocated sub-area on the target. Where a very large number of beams are used, these sub-areas are very small.
As each beam scans across the surface of the target, it is modulated in some way to reproduce the required pattern to be exposed onto the target. In a maskless lithography system, exposure data is used to modulate the beams. The exposure data usually includes pattern data describing the shapes (called features) to be exposed onto the target. As each beam scans over a certain part of the surface of the target, the pattern data may be streamed to the lithography system and used to adjust the intensity of each beam as it scans the target. For example, the pattern data may be used to switch each beam on and off to expose certain parts of the target where a feature is to be formed on the target and not expose other parts along the scan line followed by the beam.
The exposure data may also include exposure dose values, providing further modulation of the intensity of the beams as they scan. For example, if the pattern data is used to switch a beam on over a certain portion of its scan path, the exposure dose values may instruct the lithography system to set the intensity of the beam at some value between zero and one hundred percent, e.g. at 70%, during that portion of its scan path. In a raster scan lithography system this exposure dose modulation may be accomplished by dithering the beams, e.g. switching the beams on and off with a certain mark-space ratio to achieve the desired beam intensity. Note that the exposure dose values may also be used to set the intensity of a beam which the pattern data indicates should be switched off. An example of the processing of exposure data for use in a maskless lithography machine is described in E. A. Hakkennesa et al., Demonstration of Real Time pattern correction for high throughput maskless lithography”, Proc. of SPIE, Vol. 7970, 79701A-1 (2011).
The lithography system which generates the beams inevitably suffers from various errors and uncertainties arising during its manufacture and operation, resulting in errors and uncertainties in the exact position of each beam scanning the target relative to other beams scanning the target. The target is usually mounted on a stage which moves during the exposure, and there are also errors and uncertainties in the stage movement and in the exact position of the target in the lithography system. As a result, a sub-area of the target scanned by one beam may not be perfectly aligned with an adjacent sub-area of the target scanned by another beam.
To avoid exposure errors caused by this mis-alignment such as gaps between sub-areas or imperfect alignment of features exposed in adjacent sub-areas, the lithography system may be designed so that adjacent sub-areas overlap. In the area where adjacent sub-areas overlap, more than one beam may write onto the target in a so-called stitching region. Stitching in this context refers to beam writing onto the target at the interface between adjacent sub-areas. Various stitching techniques are possible for writing in the overlap area.
In one approach, one of the beams is switched off when scanning in the overlap area, so that only one of the beams is used to actually expose the overlap area. If this approach is used without regard to the pattern to be exposed, it will merely remove the overlap and re-introduce the problem of an unexposed gap between sub-areas. A “smart boundary” technique may be used in which the beam to write in the overlap area is selected based on the features to be exposed in the overlap area, to ensure that critical parts of the pattern are exposed by only one beam and any gap occurs in a non-critical part of the pattern. In another approach, both beams are used to expose the overlap area using a reduced exposure dose. One beam may fade out as it scans across the overlap area while a second beam fades in to produce a soft edge. A combination of these two approaches may also be used, in dependence on the topology of the features to be exposed.
Although these techniques can reduce the impact of imperfect alignment between neighboring sub-areas, they require complex calculation in dependence on the topology of the features to be exposed and may still result in a significantly higher variation in critical dimension (CD) in the stitching region than in other regions of the target.
In a different electron beam lithography technique, known as cell (or character) projection electron beam lithography, stitching defects may occur between neighbouring shot areas. US 2004/0191643 A1 and Hiroshi Yamashita et al., “Recent Progress in Electron-Beam Cell Projection technology” Jpn. J. Appl. Phys. Vol. 35 (1996) pp. 6404-6414 approach this problem by using additional patterns to reduce errors between adjacent, non-overlapping shot areas by using overlapping or coupling patterns. This is however a rule-based mechanical approach, realized by adapting the cell projection stencil used for forming the shaped beam.
In cell projection technology it is further common to perform a plurality of passes over the target surface in order to average out precision errors in the system. This however reduces the throughput of such systems. US 2010/0055587 A1 discloses using overlapping shots and variable doses in order to reduce the number of shots necessary for forming a pattern using the cell projection technology.
These disclosures, however, do not offer any solution to reducing the stitching errors in multiple charged beam direct write lithography described above.
US 2005/0211921 A1 and US 2012/0286170 A1, assigned to the present applicant, disclose reducing beam deflection errors in multiple beam lithography by using partially overlapping writing areas.